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BIOL 4260 Midterm: HA_SG_3.1

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BIOL 4260

Chapter 22: Heart Conduction System and Cardiac Cycle Electrical Events of the Heart • Heart depolarizes and contracts without nervous system stimulation, although rhythm can be altered by the ANS Setting the Basic Rhythm: The Intrinsic Conduction System • Coordinated heartbeat is a function of o Presence of gap junctions o Intrinsic cardiac conduction system ▪ Network of non-contractile (autorhythmic) cells ▪ Initiate and distribute impulses to coordinate depolarization and contraction of the heart • Action Potential Initiation by pacemaker cells o Cardiac pacemaker cells have unstable resting membrane potentials called pacemaker potentials or prepotentials o Three parts of AP ▪ 1) Pacemaker Potential: K+ are closed, but slow Na+ channels start to open, causing interior to become more positive ▪ 2) Depolarization: Ca2+ channels open, allowing influx of Ca, leading to rising phase of AP ▪ 3) Repolarization: K+ channels open, allowing efflux of K+ and cell becomes more negative ▪ • Sequence of Excitation- cardiac pacemaker cells pass impulses, in following order, across heart in .22 seconds o Sinoatrial node atrioventricular node atrioventricular bundle right and left bundle branches Subendocardial conducting network (purkinje fibers) o 1) Sinoatrial node- Pacemaker of heart in right atrial wall ▪ Depolarizes faster than the rest of the myocardium ▪ Generates impulses about 75x per minute (sinus rhythm) ▪ Inherent rate of 100x per minute tempered by intrinsic factors ▪ Impulse spread across atria, and AV node o 2) AV node- in inferior interatrial septum ▪ Delays approximately 0.1 second ▪ Because fibers are smaller in diameter, have fewer gap junctions ▪ Allows atrial contraction before ventricular contraction ▪ Inherent rate of 50 x per minute in absence of SA node input o 3) AV bundle (bundle of His) ▪ In superior interventricular septum ▪ Only electrical connection between atria and ventricles • Atria and ventricles not connected via gap junctions o 4) Right and Left Bundle Branches ▪ Two pathways in interventricular septum ▪ Carry impulses toward apex of heart o 5) Subendocardial Conducting Network ▪ Also referred to as Prukinje fibers ▪ Complete pathway through interventricular septum into apex and ventricular walls ▪ More elaborate on left side of heart ▪ AV bundle and subendocardial conducting network depolarize 30x/ minute in absence of AV node input ▪ Ventricular contraction immediately follows form apex toward atria ▪ Process from initiation at SA node to complete contraction takes about 0.22 seconds • Intrinsic cardiac conduction system and AP succession during heartbeat • • Clinical- Homeostatic Imbalance o Defects in intrinsic conduction system may cause… ▪ Arrhythimias: irregular heart rhythms ▪ Uncoordinated atrial and ventricular contractions ▪ Fibrillation- rapid, irregular contractions • Heart becomes useless for pumping blood, causing circulation to cease; may result in brain death • Treatment: defibrillation interrupts chaotic twitching, giving heart clean slate to start regular, normal depolarization’s o Defective SA node may cause ectopic focus, an abnormal pacemaker that takes over pacing ▪ If AV node takes over, it sets junctional rhythm at 40-60 beats/ min ▪ Extrasystole (premature contraction): ectopic focus of small region of heart that triggers impulse before SA node can, causing delay in next impulse • Heart has longer time to fill, so next contraction is felt as thud as larger volume of blood is being pushed out • Can be excessive caffeine or nicotine o To reach ventricles, impulse must pass through AV node o If AV node is defective, may cause a heart block ▪ Few impulses (partial block) or no impulses (total block) reach ventricles ▪ Ventricles beat at their own intrinsic rate • Too slow to maintain adequate circulation ▪ Treatment: artificial pacemaker, which re-couples atria and ventricles • Modifying the Basic Rhythm: Extrinsic Innervation of the Heart o Heartbeat modified by ANS via cardiac centers in the medulla oblongata ▪ Cardioacceleratory Center- sends signals through sympathetic trunk to increase both rate and force • Stimulates SA and AV nodes, heart muscle, and coronary arteries ▪ Cardioinhibitory Center- parasympathetic signals via vagus nerve to decrease rate • Inhibits SA and AV nodes via vagus nerve • Autonomic Innervation of the Heart o • APs of contractile cardiac muscle cells o Contractile muscle fibers make up the bulk of the heart and are responsible for pumping action ▪ Different from skeletal muscle contraction; cardiac muscle APs have plateau o Steps involved in AP
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